Preheat the oven to 375°F.
In a large oven-safe skillet over medium-high heat, cook the ground beef and diced onions together, breaking the beef apart as it browns, until fully cooked through. Drain excess grease directly from the skillet.
Add a splash of Worcestershire sauce and stir to combine.
Stir in the cream of mushroom soup and frozen mixed vegetables until completely combined and the soup is fully incorporated throughout.
Smooth the filling into an even layer in the skillet. Arrange the frozen tater tots in a single tight layer directly over the filling, covering it completely from edge to edge.
Transfer the skillet to the oven and bake at 375°F for 35 minutes until the tater tots are golden and crispy and the filling is bubbling around the edges.
Scatter the shredded cheddar over the tater tots and return to the oven for 3 to 5 minutes until fully melted.
Top with sliced green onions and serve directly from the skillet.